Patent Application
20080128103
The present invention relates to a method according to the preamble of Claim 1 for processing a fibre web, which method includes the spreading of a size mixture, a coating, or a corresponding processing mixture on a board web in particular. The invention can also be applied to paper webs and other similar fibre webs.
The invention also relates to an arrangement intended to apply the method.
The application relates to sizing, pigmentation, and coating and the processing of a web by calendering. Hereinafter the invention is described using as an example sizing in particular, which is the most preferred application of the invention. The invention can, of course, also be applied to the other purposes referred to above.
The term size usually refers to a water-starch mixture, which is spread on the surface of board or paper and is pressed inside the web, for example, in a nip between rolls. Size is used to increase the strength of the web. Size also improves the surface properties and printability of paper. In addition to starch, the size mixture can also include many additives while in some cases latex can also be used as the binder in the size. In principle, the present invention can be applied to all size mixtures and to different kinds of fibre web. The strength of the product is one of the most important properties of some board grades, such as the liner and fluting used in the manufacture of corrugated board. Because the use of size can, among other things, considerably increase the bending and surface strengths, the aim is to use large amounts of size in these board grades. At present, a greater application amount are achieved using pond-application devices, but their greatest running speed is too low for modern requirements. Therefore in modern board machines film-transfer application used to spread the size. In film-transfer application the size is spread first of all on the surface of a film-transfer roll, by means of which it is transferred to the web. In board machines, simultaneous two-sided application is often used, in which case the web travels through a nip formed by two film-transfer rolls. Each roll has its own application beam. Using these devices, it is possible to achieve a reasonably large application amount on the surface of one web, expressed as wet film of 30-35 g/m2.
The manner of using a coating mixture differs from sizing in that the aim is to make the solid particles of the coating mixture remain on the surface of the web.
Film-transfer coaters have, however, limitations, which reduce their efficiency and economy. Thanks to the nip pressure obtained in the nip of the film-transfer rolls, good size penetration into the fibre structure of the web is obtained, but the length of the nip is, however, quite short, so that the actual absorption of the size into the web only takes place after the nip. Because in practice the drying of the size must be started immediately after the nip, so that the web is made touch dry and can be controlled, the total absorption time remains short. Even though even large amounts of processing agent can be spread on the web using a film-transfer spreader or spreaders, a pond can arise between the surface of the web and the surface of the film-transfer roll. The creation of the pond causes the detriment of splashing at high web speeds, but a pool can arise easily even at low speeds. The creation of this so-called mini-pond limits the greatest application amount, because the pond causes splashing, web vibration, and when it rapidly rotates even the penetration of air through the nip. In a film-transfer nip, the web becomes wet with the great amount of size, which leads to runnability problems. When it becomes wet, the web weakens so that it can tear more easily. In addition, the wetting and drying change the dimensions of the web and the changes in dimension must be taken into account by adjusting the web tension and possibly by using spreader rolls. The application chambers of film-transfer coaters are designed quite precisely, to ensure an even spread of size. However, material continually detaches from the surface of a fibre web, which can travel either directly on the surface of the film-transfer roll, or along with the machine circulation into the application chamber. This causes disturbances, particularly when it gets under the doctor rod and can even block the application chamber. Further, the doctor rods and their cradles bounding the application chamber, as well as the roll surfaces wear rapidly, which leads to short service intervals. This causes direct maintenance costs and a reduction in production due to service breaks. These problems are emphasized in machines using recycled fibre, because there are always foreign substances in the fibre that cause wear, the most important of which being sand and particularly abrasive pigment-coating particles. Because recycled fibre is an important raw material in the manufacture of board, these problems become emphasized especially in machines used in the manufacture of board. During application, air and impurities can easily enter the machine circulation of the coater, and must be removed by efficient air removers and filter devices. These devices are expensive and the necessary machine circulation becomes long and its capacity large.
The problems and limitations appears in film-transfer application have different emphases in the manufacture of different products. It is obvious that the danger of a web break will not be as great in board grades as it will be with thinner papers, possibly made from recycled fibre. On the other hand, when making printing paper or board, there is less dirtying and wear of the devices.
The invention is intended to create a method and apparatus by which it is possible to is spread processing agent on a moving fibre web an amount that can be varied in wide range, the agent being advantageously surface size or pigment coating and to avoid the limitations present in application by conventional film transfer- or pond press sizer.
The invention is intended to create a method and arrangement, with the aid of which separate drying after application and the following pressing in a nip can be avoided.
The invention is based on the processing-agent mixture being spread on at least one surface of the web using a spray-application device, after which the web is led into a nip between two moving surfaces at a distance from the spreading point. Further, according to one embodiment of the invention the web is led after application to a nip, in which at least one surface that is in contact with the web is heated.
More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 1.
The arrangement according to the invention, in turn, characterized by what is stated in the characterizing portion of Claim 17.
Considerable advantages are gained with the aid of the invention.
The most important advantage of the invention is the variable amount of application over a very large area. In particular, the large application amount makes the method according to the invention more advantageous than previous methods. Thanks to the long delay time, even a large amount of size mixture has time to set on the surface of the web and inside the web and not to be pressed out in the nip of the rolls. In principle, with the aid of the method it is possible to use as large an amount of size are desired, at least within the scope of the limitations of existing known sized products. If necessary, a large application amount as permits the manufacture of new types of products. The amount of size and the profile can be adjusted with the aid of the feed amount of the nozzles or by changing the nozzle size. The delay is adjusted simply by altering the distance between the application unit and the press rolls. The final adjustment of the penetration of the size is made by altering the nip pressure of the press rolls' nip.
Preferably application is performed simultaneously on both sides of the web, in which case only one application station and the dryers it requires will be needed. There is no problem in spreading different amount of size mixture on each side of the web or using different processing-agent mixtures on each side of the web. The method also permits double application directly onto the wet surface. This is carried out by the roll nip being formed in the normal film-transfer-unit manner, in such a way that the press rolls have conventional application beams, so that a second layer of size is transferred to the web in the roll nip. The processing-agent mixture can be spread hot on the surface of the web, in which case it will be more fluid than usual and dry and set more quickly on the surface of the web. The evaporation of water is also promoted by the fact that the web can brought hot to the application device directly form the dryer.
The calender nips can also be used as the roll nip, for example, in such a way that one of the rolls is a soft roll and the other a heated roll. Correspondingly, one or both of the rolls can be replaced with belts, or the nip can be formed from a shoe calender. Thanks to the heated roll or belt and the long, preferably adjustable delay, drying after pressing can be eliminated, if the drying effect transferred in the nip to the web is sufficient. In that case, it is possible with the aid of the invention to substitute application, nip pressing of the application, calendering, and drying with only application and a hot calender nip. In particular, the expensive floating or infrared dryers are then eliminated and the purchase cost of the system can be reduced and it can be fitted into a small space, which is advantageous particularly when rebuilding a manufacturing line. This solution is best suited to pigment application, because in the case of a size with a starch content the adhesion of the size to the hot roll or belt surfaces can prevent the use of the method. In connection with sizing, particular attention must be paid to the selection of the materials of the hot surfaces, so that adhesion will be avoided.
In addition to the above advantages, the weaknesses of sizing with a film-transfer press are avoided, these being the insufficient penetration of the size to the web, i.e. limitations in the strength of the product, getting a large amount of size through the film-transfer nip, i.e. the avoidance of too great a pond forming in the roll nip, and the rapid wear of the application rods and rod cradles and roll surfaces, particularly in recycled-fibre machines. Because there are no application chambers, blockage of the chambers with material detaching from the web is avoided. Material detaching from the web does not enter the machine circulation, so that the processing-agent mixture need not be filtered in order to remove impurities. A return circulation is usually not even needed, because all of the application mixture can be directed to the web. If excess processing agent must be collected from the rolls or the spray chamber, it can be led in a simple manner back to the feed tank. The amounts are then small and the necessary transfer devices cheap. In addition to these, a pond presses have the weakness of the speed limit imposed by splashing from the pond, which limits the possibility to use them at the web speeds demanded nowadays.
In the following, the invention is examined with the aid of examples and with reference to the accompanying drawings.
The sizing station according to the invention is shown in
The application device 3 comprises a press rolls 8 mounted in bearings in a frame 7, and which form a nip, through which the web 1 being processed travels. The rolls 8 and the frame 7 can be parts of a conventional film-transfer coater, so that in this respect the application device corresponds to a film-transfer coater, from which the application devices of the rolls 8 have been removed. The web 1 is guided to run directly downwards to the rolls 8 through web guide rolls 9. The area between the web guide rolls 9 and the press rolls 8 is enclosed by a casing 10 on both sides of the web. The web 1 thus travels in a space enclosed by the web guide rolls 9, the press rolls 8, and the casing 10. The edges of the web 1 and the possible gaps can be sealed using rubber or sheet-metal sealing strips. The necessary number of spray nozzles 11 for spreading a size mist on the surface of the web are located inside the casing 10. Pressure-dispersion nozzles are preferably used as the spray nozzles 11, by means of which an even coverage is obtained while there is no drying effect of a dispersing gas. The pressure of the nozzle can be quite low, usually less than 1 Mpa, usually about 1-10 bar. However, the water content of the sizing mixture is so great that, for example, the dispersing air is not able to detrimentally dry the size mixture inside the casing, in which the humidity is high. If necessary, water vapour can be used as the dispersing gas in the nozzles, in which case drying will not take place at all while the size mixture can be simultaneously heated. In this embodiment, a size recovery system is not required, because the size mixture than condenses in the chamber on the walls of the casing or the mist in the chamber will drop or flow onto the press rolls 8 and from there onto the web 1. The walls of the casing 10 can be heated, in which case the size mist that adheres to the walls will not dry. If the size mixture flowing onto the press rolls 8 causes faults in the web 1, the excess size mixture can be collected, for instance, using channels in the lower part of the casing 10 and led back to the machine circulation or to the pulper.
If the web is treated with a pigment coating, the drying of the coating mixture may be a greater problem when using air-dispersion nozzles. Thus it is even more advantageous to use pressure-dispersion nozzles with a coating mixture. It is then also preferable for the surfaces of the application chamber not to be heated, but on the contrary to be cooled, in order to prevent the drying of the pigment coating. The fluidity of a starch-size mixture increases as the temperature rises, whereas the drying of pigment coating containing a great amount of mineral-based solids is quite rapid, if water can evaporate from the mixture into the environment.
If necessary, several rows of spray nozzles can be used. In that case, if one nozzle row is out of operation, for example during washing, running can continue using the other nozzles. The device according to the invention is most advantageous when used in two-sided coating, but naturally application on only one side can be used if desired. During web breaks and other production breaks, the nozzle beam turns away from the casing to the wash trough and if required, for example, through 180E or 90E, so that the nozzles can be checked, washed, or replaced. The walls of the casing can preferably be opened for washing.
In the embodiment described above, the delay time can be adjusted by altering the position of the nozzle rows in the casing 10. In that case, however, the casing will become large and the adjustment distance usually short.
In the solutions according to
According to the invention, the delay is adjusted in order to form the desired amount of processing agent on the web after the nip. This adjustment can take place by altering at one time the amount of processing agent or other manufacturing or quality factors of the product, such as the web speed, the material of the base web, or similar. Alternatively, the adjustment can be made continuously on the basis of measurement. The adjustment can be based on the measurement of the amount of process agent after the nip, the profile, or the quality of the surface, or even on adjustment that takes place according to the web speed.
In the nozzle casing 10, there are nozzle beams 14 on both sides of the web 1. The teachings presented in the above examples apply to the operation, location, and maintenance of the nozzles 10. However the construction of the nozzle casing 10 is different, because the rolls 8, 10 do not seal the inside of the casing 10 from the environment. The web 1 comes to the casing 10 through a narrow entry gap 15. The air that is transported with the web 1 forms a high pressure in the narrow gap and prevents the size mist from entering the casing through it. Air does not significantly enter the casing through the narrow gap 15, nor is the operation of the nozzles disturbed. In this case, the nozzles 11 are oriented in the same direction as the web 1, so that the spray 16 of the nozzle will be pulled along with the air transported by the web 1 and will come onto the web 1. In the trailing direction of the direction of travel of the web 1 there is also a narrow exit gap 19. This gap 19 is bounded on both sides by heated chambers 17. The task of these chambers is to prevent the size mixture from drying in the area of the trailing direction of the web 1. Instead of chambers, a heated plate or even an unheated surface can be used. The excess processing-agent mist is removed through outlet channels 18 arranged in the sides of the heated chambers. If necessary, vacuum can be used to remove the mist, or a small excess pressure can be created inside the casing with the aid of the air brought by the web. Air can be blown into the exist gap on the trailing side of the web, for example, using a device like an air doctor, in order to prevent the mist from escaping. According to this embodiment, the separate size-spreading device can be arranged to operate in any position of the web at all, so that the threading of the web 1 is quite free, as long as there is a sufficiently long straight section before the press rolls.
Within the scope of the invention, the mixture spread on the web 1 should be wet when it arrives at the press-roll nip. In this case, the term wet refers to the fact that with the aid of the nip pressure it will still be possible to affect the penetration of the size into the web, i.e. the size mixture can be pressed inside the fibre layer. Otherwise there will be no benefit from the nip. With the aid of the distance between the spreading of the size mixture and the press nip and the press-nip pressure the penetration into the web 1 is adjusted as desired. The size mixture can of course also be spread on the web at a fixed distance from the nip, but in that case a substantial part of the adjustability will be lost. When it is made, the size mixture is usually hot, about 50-85° C., and the size can be heated in order to maintain it at that temperature, either using a heat exchanger prior to the size being fed to the nozzles, or even electrically in the nozzle, or even with the air of a circulating heating medium in the nozzle beam. The heating of the size will reduce its viscosity, improve drop formation in the nozzle, and promote the evaporation of water on the surface of the web. Because the web comes from the final roll of the dryer-roll group, it is usually quite hot, so that the water will effective be evaporated from the hot size mixture striking the hot web surface. When using pigment coatings, it is also possible to use a heated mixture and it is often preferably for water to be effectively evaporated from the coating mixture when it transfers to the hot web. The use of a hot coating mixture may, however, cause undesired drying of the coating mixture onto the walls of the application chamber, so that when using hot coating mixtures particular care must be taken to prevent drying and to keeping the application chamber clean.
Instead of the rolls of a film-transfer press being used in the method, it is possible to use for instance a calender nip, in which one roll has a soft surface and the other roll is a heated hard roll. Both rolls can also be soft or hard and one or more rolls can be heated. If the rolls are different to each other, two roll nips may be needed. In that case, however, the effect of the later roll nip on the penetration of the processing agent will be small, because the surface of the web may become to dry after the first roll nip. Instead of a roll nip it is possible to use a belt or shoe calender, which comprises one or two consecutive nips. If a metal belt is used in the calender, the belt can easily be heated or cooled to alter the surface properties of the web.
The invention is highly suitable for use in rebuilding or replacing an existing pond size press or a film-transfer press. In that case, the technology required to form the pond is removed, or correspondingly the application chambers and coating or size circulations. These application methods are replaced according to the invention by placing a spray-application beam before the roll nip. A rebuild of this kind is relatively cheap and with its aid the running speed and amount of processing-agent used can be substantially increased. Thanks to the greater amounts of processing agent, the device can be used to manufacture a wider product range than previously.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20041557 | Dec 2004 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI05/00522 | 12/1/2005 | WO | 00 | 5/30/2007 |
